1. Field of the Invention
The present invention relates to carbon black for coloring cement which is suitable for coloring a cement-formed product in black, and to a method for coloring a cement-formed product using the carbon black.
2. Description of Background Art
As a coloring agent for coloring various types of cement-formed products used for buildings, inorganic and organic pigments have been conventionally used. As black pigments, iron oxide (iron black) and carbon black are used.
Iron oxide has good coloring characteristics, but exhibits poor corrosion resistance, in particular, inferior acid resistance. For example, iron oxide is easily discolored due to acid rain or the like, thereby causing a decrease in weatherability of the cement-formed product. Moreover, it is necessary to mix a large amount of iron oxide with the cement-formed product in order to increase the degree of blackness. As a result, the amount of water to be added when mixing a dispersion solution of iron oxide with cement slurry is also increased, whereby the strength of the cement-formed product is decreased.
On the contrary, carbon black is not discolored due to acid rain or the like, thereby differing from iron oxide. Moreover, since the carbon black has good coloring characteristics, it suffices to mix only a small amount of carbon black with cement components. For example, Japanese Patent Application Laid-open No. 3-60483 discloses a cement-formed product in which the surface of a formed product obtained by forming a cement forming material including a black coloring agent is covered with a black coating film. This application discloses carbon black as the black coloring agent.
However, since the carbon black has low dispersibility in water because of its surface characteristics, it is very difficult to homogeneously and stably disperse the carbon black in the cement slurry. Moreover, since the carbon black is in the form of very minute powder, the carbon black inhibits the hardening of the cement. Furthermore, the carbon black separates from the cement matrix after hardening with the passage of time due to low adhesion to a cement matrix. This causes a decrease in the strength of the cement-formed product.